Fix __iter__ return type to Iterator

[altescy]

This pull request modifies the type hint for __iter__ to return Iterator instead of Iterable ¹.
Mypy raises the error if __iter__ returns Iterable like below:

from typing import Iterable

class Foo:
    def __iter__(self) -> Iterable[str]:
        return iter("abc")

for value in Foo(): # E: "Iterable[str]" has no attribute "__next__"  [attr-defined]
    ...

Footnotes

1. iterator.__iter__ ↩

[cla-checker-service]

💚 CLA has been signed

[pquentin]

Thanks! Great catch. LGTM.